Security alarm systems are used in homes and in commercial and industrial facilities, for monitoring the premises to detect intruders, fire and environmental hazards such as carbon monoxide contamination. There are two types of security alarm systems: wired and wireless. Although both have been proven effective over many years, security alarm systems are still utilized in a minority of premises because of the cost and limitations of conventional wired systems.
Much of the cost of installing a wired alarm system is in the wiring, since the peripheral devices such as sensors and alarm condition indicators are dispersed throughout the premises and thus well remote from the base unit. In general it is advantageous to divide the premises into “zones” which are monitored independently, and although the sensors within a zone can be wired in series the zone circuits must be connected to the base unit in parallel in order for the base unit to be able to discriminate between zones.
A lot of wiring is required to adequately cover vulnerable entry points divided into a desirable number of zones, and a service technician installing such a system must thus spend considerable time fishing wires through walls in order to make the alarm system unobtrusive. This is a time consuming and costly procedure, and it is not always possible to conceal the wiring in structures such as condominiums and apartment buildings, older houses and houses with finished basements.
Wireless alarm systems are also known. In these systems a plurality of different kinds of wireless sensors distributed about the premises each emit a radio frequency (RF) signal with a characteristic frequency. The signal is transmitted when the sensor detects an intrusion, for example a point contact which is broken when a window is forced open or a motion detector which senses motion within its detection field. A receiver contained in a central unit monitors the various RF signals and signals a controller when one or more of the signals is transmitted, indicating an alarm condition. Such a system may provide many types of peripheral surveillance devices, including door and window sensors, motion detectors, “panic” transmitters and glass break detectors; peripheral environmental devices, including detectors for sensing smoke, combustion gas, carbon monoxide, moisture and temperature changes.
Wireless alarm systems are considerably easier to install, because peripheral sensors and actuators need only be mounted and do not have to be wired to the control panel. However, each wireless sensor and actuator requires a separate channel for communications to the control panel, in order to avoid “collision” between competing RF signals from different peripheral devices and to allow the control panel to recognize which peripheral sensor is emitting the signal. In a typical wireless system there are a limited number of RF channels available for peripheral sensors, and the cost of the system increases considerably as more channels are added.
Further, conventional systems provide only one-way communication, i.e. sensor-to-control panel signals, and thus alarm indication peripherals such as audible indicators and line seizing devices must still be wired to the base unit, which increases installation costs. In some cases the RF signals emitted by the peripheral sensors have a limited range before they become subject to interference and unreliability, which can limit the location of such peripherals and/or the control panel, or require expensive repeaters which receive the signal from a remote peripheral device and relay the signal to the control panel.
Also, the one-way communication of such systems precludes any verification procedure, which can result in false alarms caused by equipment malfunction and lead to costly and unnecessary response by emergency services personnel. This substantially reduces the reliability of the security system. Reliability is critical in a security system, and a wireless security system that cannot provide extremely high reliability is commercially unfeasible.
Patent Cooperation Treaty Application No. PCT/CA00/00662 filed Jun. 6, 2000 by the present applicant, which is incorporated herein by reference, describes a wireless security alarm system providing reliable two-way communication between a base unit and a plurality of peripheral devices, including both sensors and alarm indicators. This system provides a large number of channels for monitoring both intrusion and environmental conditions. The peripherals used in this system can be configured and reconfigured through the control panel, which has a digital communicator having full upload and download capabilities, permitting remote programming, alarm reception and verification and analysis of an alarm condition.
The use of wireless peripherals increases the possibility of intentional signal jamming and interference from other wireless systems, and miss-detection due to multi-path phenomena (where a transmitted signal is received both directly from the transmitting peripheral and indirectly through reflection off of the ground, the protected structure and other objects). These problems commensurately reduce the reliability of the security system. Where a large number of wireless peripheral devices are involved, opportunities for collision between peripheral transmissions is increased, further reducing the reliability of the security system.
Because of these potential problems, and because in a conventional wireless security system communication is one-way and the control panel cannot communicate with the peripheral sensors to acknowledge reception of a peripheral transmission, to obtain any reasonable level of reliability conventional wireless peripheral devices are designed to repeatedly signal the control panel in order to maximize the probability that the control panel receives the transmission. This redundant emission of RF signals further exacerbates the opportunities for collision mentioned above, and significantly reduces the operating life of the batteries used to power the peripheral devices.